Conventionally, in a case where high feed machining, in which a feed per revolution exceeds 5% of a drill diameter, is performed with a drill with a linear cutting edge, fracture or the like of the cutting edge is likely to occur due to insufficient strength of a corner portion of the cutting edge, so the machining may be prevented in some cases. Furthermore, even in a case where the drill is used under normal conditions in which the feed per revolution is 2 to 3% of the drill diameter, the corner portion of the drill cutting edge may often be fractured or completely worn out to reach durability limit.
Therefore, Patent Document 1 proposes a drill which has a cutting edge with fracture resistance and which enables favorable chip curling as well as deep hole drilling and through hole drilling in a stable manner. In the drill, a portion, which is closer to a center cutting edge portion, of a main cutting edge portion is formed in a straight line or a convex arc with a large curvature radius. Furthermore, the outer peripheral side is formed as a convex-shape curved edge having a curvature radius that is gradually reduced towards the outer periphery. The lower limit of the curvature radius is 0.25 times the drill diameter. Moreover, the main cutting edge portion has no inflection point and a radial rake angle q at the outer peripheral portion is less than or equal to −15 degrees and greater than or equal to −40 degrees. The drill having a cutting portion formed as above disperses load across the main cutting edge, thereby preventing concentration of the load at one point. As a consequence, edge fracture is unlikely to occur, thus the tool life can be lengthened.    Patent document 1: Japanese Patent Application Publication No. 2003-285211